This invention relates to a forming process for forming, by forging, an internal gear profile cup-shaped member such as an outer casing of hydraulic pressure operated clutch or the like, and an apparatus for the same.
As for a process of this kind, there has been hitherto known a push-in forming process such as disclosed in Japanese Patent Application Sho No. 58-48258. In this process, a gear profile punch is pushed into an opening of a cup-shaped raw workpiece to contact the inner surface of the circumferential panel thereof and form it. An extrusion process is known where a solid raw workpiece is extruded to protrude rearwards by a gear profile punch. An ironing process is known where an outer surface fo the circumferential panel of a cup-shaped raw workpiece is ironed.while an inner surface of the circumferential panel is restrained by a gear profile punch.
However, the push-in type process has problems in that there is a restriction in gear shape usable. In addition it is extremely difficult to obtain an effective forming of the internal shape extendin9 all of the way to the bottom surface of the cup-shaped member. In the push-in type process, the punch suffers a remarkable amount of abrasion and is short in service life. The extrusion type process has problems in that a high forming load is required. Especially, a gear forming work on a large diameter solid raw workpiece by cold forging is disadvantageous in that not only a large-sized pressing machine is required, but also a stress involved is so large that abrasion of the punch is further increased. The ironing type process has no problems with respect to abrasion of the punch but has problems especially in such a case where the internal gear profile punch is large in size and high in height, the movement of the metal of the raw material of the workpiece caused by outer circumferential ironing tends to be exclusively in the vertical direction, and not in the circumferential direction, so that a perfect gear-shape forming is extremely difficult to obtain.